Tube amp startup hum

[Miyagi_83]

Hello everyone. I just finished building a new amp. Push-pull 6F3P with a 6N2P & 6ж8 preamp. It's working as I wanted it to, but there's one thing that's bothering me, namely, when I start it up, after about 15-20 seconds there's an audible hum which then gradually disappears. I checked all ground connections, I tapped on every component inside - nothing.
You can hear what I mean here

https://soundcloud.app.goo.gl/UJwSc

What are the odds that's normal behavior for some tubes? Or is it a bad cap?
Thanks in advance for any help.

[PRR]

Slow bias supply?

[Miyagi_83]

Thanks for the reply, PRR. I was hoping you and/or some other high class experts would chime in. This one is cathode biased.

[sluckey]

It's possible that even two matched output tubes warm up at different speeds. This could cause hum that disappears once the tubes are settled in.

[Miyagi_83]

Thanks for your reply, Sluckey. I guess I have nothing to worry about then.
None of my previous builds, push pull or single-ended, had done that. Hence my question.
I'll buy another matched pair and see if it continues to behave like that. I should report back within a few days.

[Latole]

Preamp tube ?

[pdf64]

It shouldn’t be too hard to monitor for current imbalance between the push pull sides as the amp warms up.
It seems pointless getting another matched pair. It would be pure luck whether they behaved any differently than the pair you’ve got.

[PRR]

...I was hoping some high class experts would chime in....

OK.

[Miyagi_83]

Preamp tube ?

Thanks, Latole. I'll certainly check that out. Somehow it didn't cross my mind.

...I was hoping some high class experts would chime in....

OK.

Did I say something wrong? 

It shouldn’t be too hard to monitor for current imbalance between the push pull sides as the amp warms up.

Thanks, pdf64. I'll try to do that as soon as possible.

[Latole]

You will have to try other tubes, a lamp tester cannot detect this problem.

The volume turned to 0, do you hear the hum?

[HotBluePlates]

... I was hoping some high class experts would chime in. ...

OK.

I think he meant that your reply fulfilled his hopes.

[shooter]

 

[66Strat]

Inflection is everything. 

[Miyagi_83]

Preamp tube ?

You will have to try other tubes, a lamp tester cannot detect this problem.

The volume turned to 0, do you hear the hum?

Preamp tubes out, the hum's there. Volume at 0, the same. Once I have some more time I'll try to measure current imbalance, as suggested.

[Latole]

It is not a preamp tubes issue

[pdf64]

...I'll try to measure current imbalance...

One method would be to fit a 1ohm cathode resistor per cathode, ie between cathode and a common point eg 0V common, shared cathode bias resistor.
Then monitor the mVDC discrepancy between the cathodes, as the valves warm up.

[Miyagi_83]

One method would be to fit a 1ohm cathode resistor per cathode, ie between cathode and a common point eg 0V common, shared cathode bias resistor.
Then monitor the mVDC discrepancy between the cathodes, as the valves warm up.

Thanks for the hint. I was thinking about measuring voltage drop across OT primaries and their DC resistance. IIRC Rob Robinette suggested that on his website.

[Miyagi_83]

... I was hoping some high class experts would chime in. ...

OK.

I think he meant that your reply fulfilled his hopes.

That's exactly what I meant

[Miyagi_83]

Thanks for the reply, PRR. I was hoping you and/or some other high class experts would chime in. This one is cathode biased.

There.

Inflection is everything. 

You're absolutely right, sir

[Miyagi_83]

Ok, so I measured the DCR of my OT's primary windings and, since the hum is present only at startup while the valves are warming up, I tried to get voltage readings from center tap to the anodes the moment the hum started to get audible. I fired the amp up once, measured one anode. Then I turned the amp off, waited for it to cool down, turned it on again and measured another anode. Now, because I only have a DMM on hand, I'm not perfectly sure I got the readings right. However, there seems to be a discrepancy between the anode currents of my output valves. Here's what I got:

RED-BROWN - 148,9 Ohms - hum came up at 0.9 V = 6 mA
RED-BLUE - 155,4 Ohms - hum came up at 1.4 V = 9 mA
and it calmed down when the current draw reached about 22 mA per valve.

If I got something mixed up, feel free to correct me.

[shooter]

those #'s look good to me.  how long have the tubes been used?
I don't even try and match my tubes til they have between 10-50 hrs of cook-in time
and 3mA is nothing, even for cork-sniffing Audio

[Miyagi_83]

those #'s look good to me.  how long have the tubes been used?
I don't even try and match my tubes til they have between 10-50 hrs of cook-in time
and 3mA is nothing, even for cork-sniffing Audio

I bought them NOS and have been using them for less than 2 hours altogether. They'd already been matched by the seller or whoever it was. At least that's what the description said. Perhaps, as Sluckey pointed out, they just warm up at different paces and there's no more to it I'm neither a professional nor a very experienced builder to tell.

I'll take the amp to band practice and try to rock the living light out of it. If it survives, which I think it will, it's going to be fine because other than that ca. 10-second-long hum at startup the amp works the way it should. I'll probably do some tweaking to the preamp but I need to have someone less deaf than me around to listen to it as I go on Should anyone be interested in the schematic, I'll have to draw it and will be more than happy to share.

Thanks for all the help, gentlemen.

[sluckey]

Might be worthwhile to temporarily add a Standby switch and start up in STBY mode. Switch to operate after about 30 seconds. Still hum?

[Miyagi_83]

Might be worthwhile to temporarily add a Standby switch and start up in STBY mode. Switch to operate after about 30 seconds. Still hum?

Not big on Standby switches, but I'll try and report back. Thanks for the suggestion.

[66Strat]

What type of rectifier is in the circuit? The start up hum would likely go away with a slow warm-up type like a 5AR4/GZ34.

[Miyagi_83]

What type of rectifier is in the circuit? The start up hum would likely go away with a slow warm-up type like a 5AR4/GZ34.

Right now there's a SS rectifier, 2x 1N4007, and I can only use rectifiers that run on 6.3V heaters because the transformer I've used doesn't have a 5V winding.

[pdf64]

EZ81 then 
But really, other than user perception, it may well be a complete non issue.

[Miyagi_83]

Might be worthwhile to temporarily add a Standby switch and start up in STBY mode. Switch to operate after about 30 seconds. Still hum?

I installed a temporary standby switch and it did the trick. When I flipped it after the valves were properly warmed up there was no hum
I'm not going to install one permanently though because I have no space. Thank you for your help.